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A close up of a topographic map of Pittsburgh Pennsylvania. Map is shades of orange and red with a river through the middle and buildings shown with black rectangles.
Sheet 1, Pittsburg West, Pennsylvania 1:25,000. Army Map Service, 1953. Geography and Map Division.

Digital Image Processing: It’s All About the Numbers

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This is a guest post by Rachel Trent, Digital Collections and Automation Coordinator in the Geography and Map Division.

Every time you look at an online image of a historical map, what you’re viewing is really just a spreadsheet of numbers. Or more likely, three spreadsheets, one each for red, green, and blue (the technical way to describe this is as a “3-dimensional array”, but it’s ok to simply think of it as three spreadsheets). Each of the image’s pixels is represented by a number from the red spreadsheet, the green spreadsheet, and the blue spreadsheet. Your device simply visualizes that numerical data as a grid of colors.

Thousands of Library of Congress maps are imaged each month, allowing you to not only view them online but also allowing you to analyze a